1. Field of the Invention
The present invention relates to a light emitting device which is excellent in color purity, color rendering properties and durability, and also to a luminescent medium where the aforementioned light emitting device is employed.
2. Description of the Related Art
Nowadays, light emitting devices using inorganic fluorescent substance are now being rapidly improved in terms of the luminous efficiency thereof. In particular, the luminous efficiency of white light emitting devices is now expected to surpass the luminous efficiency of a fluorescent lamp in the near future. However, when it is desired to employ light emitting devices in a lighting system, there are many situations where light emitting devices are required to be excellent not only in luminous efficiency but also in color rendering properties. In the case of the light emitting devices where only an inorganic fluorescent substance is employed however, it is now considered very difficult to satisfy all of these properties required in a lighting system.
The idea to employ an organic fluorescent substance for light emitting devices is well known. However, light emitting devices where an organic fluorescent substance is employed as a luminous medium have not yet been put into practical use due to the following problems.
1) Particularly in the case where a near-ultraviolet LED chip is employed as a light source as currently increasingly employed and where an organic fluorescent substance is employed for light emitting devices comprising luminous medium of R, G and B, the organic compound employed therein is caused to deteriorate considerably due to ultraviolet rays. The reason for this can be ascribed to the fact that organic compounds are generally vulnerable to ultraviolet rays. In particular, when an organic compound exhibits absorption of light based on n-n transition in the near-ultraviolet zone, the organic compound would be rapidly deteriorated.
2) An organic fluorescent substance may exhibit the fluctuation of fluorescence spectrum depending on the concentration thereof, so that it is difficult to control the spectrum thereof. Furthermore, the intensity of fluorescence of organic fluorescent substance may also fluctuate depending on the concentration thereof, so that the concentration extinction would be caused to generate at a high-concentration zone thereof.
3) The fluorescence spectrum of an organic fluorescent substance may be fluctuated depending on the kinds of polymer employed for dispersing the organic fluorescent substance.
Generally, fluorescent substances made of rare earth complexes are advantageous in the following respects as compared with the ordinary organic fluorescent substances. One example of the rare earth complexes which is useful in a lower molecular organic EL element can be represented by the following formula (6).

In the rare earth complex shown by the following formula (6), the ligand thereof is constituted by phenanthroline and β-diketone. As this phenanthroline absorbs light, an excited state thereof is caused to occur, thereby permitting energy shift to occur from the triplet excitation state to the central europium, thus making it possible to obtain a light emission of 612 nm which is peculiar to europium.
Since this rare earth complex is capable of absorbing light through phenanthroline, the absorption coefficient thereof is relatively large, thus enhancing the intensity of luminescence. The fluorescent substance including the aforementioned rare earth complex has the following advantages as compared with the fluorescent substance consisting of an ordinary organic compound.
1) The wavelength of emission is peculiar to rare earth elements and the luminescent spectrum is quite stable, i.e. free from any influence that may be caused due to differences in color concentration and in kinds of polymer dispersant.
2) Although the ligand is constituted by an organic compound, once the ligand is turned into an excited state as it absorbs light, the ligand is permitted to return to its ground state due to the shift of energy to the central element, so that the opportunity of generating an irreversible chemical change from the excited state can be minimized. As a result, the fluorescent substance is expected to have excellent resistivity to ultraviolet rays.
However, the fluorescent substance including the aforementioned rare earth complex having phenanthroline as a ligand is accompanied with problems that the solubility thereof to a solvent as well as the dispersibility thereof to a resin are poor.
Whereas, the fluorescent substance including a rare earth complex represented by the following formula (7) is excellent in solubility to a solvent and can be homogeneously dispersed in a resin.

However, the rare earth complex represented by the formula (7) is accompanied by the problems that the absorption coefficient thereof in a near ultraviolet region is relatively small, so that it is impossible to secure sufficient intensity of luminescence.
Whereas, the employment of a rare earth complex where triphenylphosphine oxide is employed as a ligand as shown by the following formula (8) is proposed (see for example, Chemistry and Industries Vol. 53, No. 2 (2000), pp. 126-130; Journal of Organic Synthetic Chemistry Association, Vol. 58, No. 10 (2000), pp. 945-955; Chem. Lett. (1999), pp. 35-36).

The compound represented by the formula (8) is higher in luminosity as compared with the compound represented by the aforementioned formula (7) but is still insufficient for use in the light emitting device, so that the luminosity thereof is required to be further improved.